KOP Lexus Owner

Has anyone had any problem with their 2010 ES Voice Recognition on the Nav? Lately, mine won't always respond to my voice. Also, at times it will say that "that function is not available at this time" (for example, if I say, Destination.) Dealership tells me that they haven't had any problems with the nav system. They ran diagnostic check today and it concluded that everything was OK. Really frustrated here.